1. Field of the Invention
This invention lies in the field of medical devices such as artificial knee and hip joints. In particular, this invention addresses matters associated with the use of ultra high molecular weight polyethylene (UHMWPE) and other biologically compatible polymers suitable for use in the manufacture of such devices.
2. Background of the Invention
Polymers are widely used as materials of construction in medical devices such as artificial joints, bio-instruments, and other medical implants. Knee joint replacements, many of which use UHMWPE as the tibia component, are examples of such devices. Unfortunately, artificial knees and other articulated implants have a limited life span in the body since the wear on the UHMWPE component causes the material to deteriorate and to form debris which leads to inflammation and osteolysis. Other factors that limit the life span of UHMWPE and other polymers used in these devices are cyclic damage, contact stresses, friction, and possibly the hydrophilic/hydrophobic character which may affect biocompatibility. Whatever the cause, the device ultimately reaches the end of its life span and a replacement is needed. Unfortunately, replacement surgery (which is termed a xe2x80x9crevisionxe2x80x9d operation) is often more difficult and poses a higher risk than the original implantation surgery. Nearly 500,000 artificial joints are implanted in the United States each year, and the average artificial joint lasts about 15 years before it must be replaced. This time span suggests that a single implantation may be suitable for older or less active patients. Young, active patients however may require one or more revisions, and the number of revisions increases with the increase in the life expectancy of the general population. Aware of the low success rate of revisions, many younger patients wait (often in pain) before their first arthroplasty operation in order to lessen the number of revisions that they will need during their lifetime.
The development of sophisticated techniques such as transmission electron microscopy for characterizing surfaces has led to an improved understanding of wear mechanisms. As the polymer is subjected to wear, the polymer delaminates and particles of the polymer separate from the component. The separated particles are then released into the surrounding tissue. Crystalline lamellae that are part of the polymer structure are particularly susceptible to the shear forces that arise when the contacting surfaces slide against each other, since these shear forces cause the lamellae to align at the surface, which increases their susceptibility to breakage. This causes further particle formation and separation.
Several theories have been advanced to explain the mechanisms by which wear occurs in the UHMWPE used in total joint prostheses. Some of these theories are described by Dumbleton, J. H., et al., in xe2x80x9cThe Wear Behavior of Ultrahigh Molecular Weight Polyethylene,xe2x80x9d Wear, vol. 37, pp. 279-289 (1976); Nusbaum, H. J., et al., in xe2x80x9cWear Mechanisms for Ultrahigh Molecular Weight Polyethylene in the Total Hip Prosthesis,xe2x80x9d J. Appl. Polymer Sci., vol. 23, pp. 777-789 (1979); and Engh, G. A., et al., in xe2x80x9cPolyethylene Wear Metal-Backed Tibial Components in Total and Unicompartmental Knee Prostheses,xe2x80x9d Journal of Bone and Joint Surgery, vol. 74-B, pp. 9-17 (1992). According to these theories, prostheses containing a UHMWPE component in articulating contact a metal or metal alloy component undergo both adhesive and abrasive wear. Material is disengaged from the surface of the UHMWPE component by asperities of the metal component or by third-body abrasion when previously separated particles are drawn across the contact interface. Additional theories cite the occurrence of surface and subsurface cracking caused by high contact stresses at the surface. Subsurface cracks propagate through the material and join other subsurface and surface cracks, leading to delamination and the deterioration of the delaminated material into particulate debris.
The particles released during the wear of UHMWPE components in total knee replacements are on the order of 1 micron in size. Particles of this size elicit an immune response in neighboring tissues. Since giant cells (macrophages) generally do not metabolize such particles, the particles remain in the physiological system and lead to chronic inflammation and pain. Fatigue due to subsurface cracks may itself lead to catastrophic failure, but fatigue coupled with wear is generally the greatest life-limiting factor. Debris from frictional sliding between the polymeric and metallic surfaces of the implant leads to clinical complications long before the materials fail due to macroscopic fatigue.
Immune reactions from particulate debris and mechanisms by which these reactions lead to osteolysis or accelerated bone re-absorption are reported by Schmalzried, T. P., et al., xe2x80x9cPolyethylene Wear Debris and Tissue Reactions in Knee as Compared to Hip Replacement Prostheses,xe2x80x9d Journal of Applied Biomaterials, vol. 5, pp. 180-190 (1994); and Lewis, G., xe2x80x9cPolyethylene Wear in Total Hip and Knee Replacement,xe2x80x9d Journal of Biomedical Materials Research, vol. 38, pp. 55-75 (1997). Osteolysis leads to degradation of the anchoring bone, making revision surgery more difficult if not impossible, as reported by Howie, D. W., xe2x80x9cTissue Response in Relation to Type of Wear Particles Around Failed Hip Arthroplastics,xe2x80x9d J. Arthroplasty, vol. 5 (1990). The effect of particles entering the lymph nodes is largely unknown.
Other investigators have examined the material properties of the femoral component and have suggested a range of possible alternative materials and surface modifications, as discussed in Ratner, B. D., et al., Polymer Surfaces and Interfaces, edited by Feats, W. J., et al., John Wiley, Chichester, UK, pp. 231-251 (1987); Davidson, J. A., et al., xe2x80x9cSurface Modification Issues for Orthopedic Implant Bearing Surfaces,xe2x80x9d Materials and Manufacturing Processes, vol. 7, pp. 405-421 (1992); and Walker, P. S., et al., xe2x80x9cWear Testing of Materials and Surfaces for Total Knee Replacement,xe2x80x9d Journal of Biomedical Materials Research, vol. 33, pp. 159-175 (1996).
Further disclosures of potential relevance to this invention are descriptions of the use of radio frequency power sources used to energize a gas to produce a plasma as disclosed in Kolluri, O. S., xe2x80x9cPlasma Surface Engineering of Plastics for Medical Device Applications,xe2x80x9d Materials Plastics and Biomaterials (1995). The effect of high concentrations of CF3 groups on the surface of UHMWPE in promoting the binding of proteins is described by Castner, D. G., et al., xe2x80x9cRF Glow Discharge Deposition of Fluorocarbon Films for Enhanced Protein Adsorption,xe2x80x9d Annual Meeting Society for Biomaterials, San Francisco, Calif., p. 218 (Mar. 18-22, 1995).
It has now been discovered that prosthetic implants with components made of UHMWPE or other high molecular weight polymers that suffer the disadvantages enumerated above can be improved by treating the surface of the polymeric component with a plasma gas to produce various conversions or modifications of the polymer at and near the surface. By appropriate selection of the plasma gas and the conditions of treatment, one can select a particular conversion or modification to address a particular problem or to benefit the polymeric component and the implant as a whole in any of a variety of ways, such as improving wear resistance, reducing the tendency toward the release of particular debris, lessening friction between the polymeric component and an adjacent component, increasing either the hydrophilic character or the hydrophobic character of the polymer surface, modifying the chemistry of the surface by attaching functional groups, sterilizing the surface, roughening the surface, or making it more biocompatible.
One conversion achievable by the practice of this invention is crosslinking of the polymer at the surface. This improves the wear resistance of the polymer by reducing or eliminating the tendency of the polymer chains and the crystalline lamellae to align at the surface and thus reducing their susceptibility to breakage into particles. Conversely, it has been discovered that crosslinking throughout the bulk of the polymer is not beneficial, since it lowers the resistance of the polymer to crack propagation and thereby renders the polymer component more susceptible to fatigue. Crosslinking in a concentrated manner at the surface, and preferably also in regions near the surface with a crosslinking density that decreases with increasing distance from the surface, thus improves the wear resistance without substantial loss of component fatigue resistance.
Other conversions achievable by the practice of this invention, either in conjunction with or independent of crosslinking, are coupling reactions between the polymer surface and the plasma gas. Included among these reactions are the covalent attachment of groups to the surface, using groups that have particular functionalities or hydrophobic or hydrophilic characteristics that benefit the longevity or utility of the polymer as a component of the implant, or the compatibility of the polymer with the surrounding tissue. The plasma reagent may thus be one that places hydroxyl groups or other hydrophilic groups on the polymer surface, or one that places hydrophobic organic groups or low-friction fluorocarbon groups on the surface. The lowering of friction achieved by the covalent attachment of fluorocarbon groups when combined with surface crosslinking is particularly effective in minimizing shear deformation, bulk fractures, and surface delamination of the polymeric component. This in turn reduces and possibly eliminates the presence of loose particles, the loosening of joints, and the re-adsorption of bone.
Plasma treatment in accordance with this invention can thus be used to modify the surface chemistry and microstructure of the polymeric component of an implant in ways that will benefit the component and the implant, and treatments producing two or more effects can be performed simultaneously or in sequence. The treatments can also be combined with additional treatments for supplementary purposes such as a preliminary sterilization of the component. These and other features, advantages, and aspects of the invention are described below in detail.